I just run a check using something like:

my @set = qw(1 2 3 4 5 6);
my @add_to_set =  qw(5 6 7 8 9);
# note dupes.........^ ^....

foreach (@add_to_set) {
  if (!is_member($_,@set) {
    push(@set,$_);
  } else {
    warn "duplicate member detected...\n";
  }
}

sub is_member {
  my $self = shift;
  my $test = shift;
  my $ret = 0;
  foreach (@_) {
    if ($test eq $_) {
      $ret++;
      last;
    }
  }
  return $ret;
}
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As far as I know, there are no native set functions like intersect, union, etc, but these can be easily created with what is available.